As the days get shorter, more runners have to train in the dark. A study published in the Journal of Sport & Exercise Physiology helps to explain the common perception that running a given pace in the dark feels harder than during daylight hours.

The study measured perceived effort during different conditions of optic flow, or your visual sensation of moving through an environment. Lead researcher David Parry, MSc, and colleagues at the University of Essex's Human Performance Unit had cyclists on stationary bikes do a 20-kilometer time trial. The cyclists then did three more hard 20K time trials, spaced out over several days. In the three subsequent time trials, the cyclists were asked to match the average cadence and power output of their initial time trial. They reported their perceived effort, or how hard their pace felt, every 4 kilometers during the time trials.

During the time trials, the cyclists watched a video of a road being traveled on. What the cyclists didn’t know was that Parry manipulated their optic flow on two of the subsequent time trials. During one, optic flow was sped up by 15%; that is, the scenery passed by 15% faster than the speed at which the cyclists were riding. On another, optic flow was slowed by 15%. After the time trials, the cyclists confirmed that they were unaware of the variations in optic flow.

Parry found that the cyclists rated their perceived effort as lower when optic flow had been slowed. At the end of the optic-flow-slowed time trial, the cyclists put their effort at 14 on the Borg scale of perceived effort, which corresponds to an effort level of “somewhat hard.” In contrast, at the end of the optic-flow-sped-up time trial, as well as the optic-flow-normal time trial, the cyclists rated their effort at 15 on the Borg scale, which corresponds to “hard” or “heavy.”

What does this have to do with running in the dark?

“An interesting implication of these findings is that different environments may create different perceptions of effort for the same running speed,” Parry told Runner’s World Newswire. “When running at night, in the dark, objects further away aren’t visible and you only have close-by objects to use as reference, and so you get a greater sense of speed compared to running during the day.” Most people who have done track workouts in the dark will agree that it seems to take more effort to hit a given time than when doing the same workout under a sunny sky.

Parry said that his research also has relevance for how running feels in different settings.

“Running in an environment where most of the visual reference points you can see are close by, you experience a greater sensation of speed than when in an environment where your reference points are far away,” Parry said.

“Running, therefore, on an open trail with expansive views across the landscape, and relatively few object close by, is likely to lead to lower sensations of speed than running in a forest with many trees nearby, or in a city with buildings and road signs close to you.”